The majority of casings of steering wheels with an integrated airbag are constructed today in two parts. A moulded part surrounding the skeleton structure of the steering wheel and with a separately produced covering for the airbag installation opening in the hub region of the steering wheel, with great efforts being made to match the two parts to each other visually and haptically. Considerable further difficulties result from the fact that the airbag covering has to be constructed so that the covering is able to be torn open and is also movable relative to the steering wheel hub by the touch stroke necessary for actuating the horn.
As a gap between the moulded part encompassing the steering wheel and the airbag covering is regarded as being visually disadvantageous, it has already been proposed to produce the moulded part and airbag covering in one piece. Owing to the tear-open and actuation functions to be taken into account in the region of the airbag coverings, such methods involve certain substantially solvable difficulties with regard to manufacturing technique and compulsorily require a largely new design of the steering wheel construction, because the installation can only take place from the rear side of the steering wheel and because special precautions have to be taken for the introduction of the airbag module with folded gas bag and gas generator. One known design involves dividing the steering wheel skeleton structure into a base part with hub and possibly spoke stumps and a steering wheel rim with spokes. The base part can be equipped with the airbag module and other functional parts, before the steering wheel rim is attached to the base part and the entire skeleton structure is covered by a one-piece sheathing.
It is regarded as particularly disadvantageous here that the fastening between base part and steering wheel rim involves an additional material expenditure and adds weight to the structure, which counters the general aim for saving weight in automobile construction. In addition, it is difficult to match the two components optimally to each other with regard to dimensions. In particular, the gas bag covering, as a moulded part, is provided with considerable tolerances. Because the gas bag covering has to be able to be torn, it must not have any particular inherent rigidity and therefore requires the folded gas bag as a support. If the gas bag covering is too "thin", the covering is not sufficiently supported, if it is too "thick", it is buckled upwards. Both are undesirable.
Similar problems result with regard to the actuation movements which have to be transferred from the exterior via the plastic moulded part, which is constructed in one piece on the steering wheel rim, onto the actuating members mounted in the base part. It should be readily clear that the adaptation of a surface, produced by casting mould technique, to a mounted surface is very costly with regard to manufacturing technique, and that the transfer functions can only be carried out reliably with regard to operation if very close tolerances can be maintained in adaptation.
Resulting from this is the problem of further developing the method initially mentioned so that all the above-mentioned difficulties can be overcome and so that a one-piece plastic part for encasing a steering wheel can be provided. with a justifiable economic expenditure.